Method for coating ferrous metal surfaces



United States Patent 3,519,494 METHOD FOR COATING FERROUS METAL SURFACESRudolf Engesser, Frankfurt am Main, Richard Tuch, Habsburger Allee,Werner Rausch, Stierstadt, Taunus, and Winfried Menzer,Sprendlingen-Hirschsprung, Germany, assignors to Hooker ChemicalCorporation, Niagara Falls, N.Y., a corporation of New York No Drawing.Filed June 21, 1967, Ser. No. 647,595 Claims priority, applicationGermany, July 12, 1966,

1,521,879 Int. Cl. C23f 7/10 U.S. Cl. 148-615 Claims ABSTRACT OF THEDISCLOSURE A process for forming a protective and/or paint-base coatingon ferrous metal surfaces wherein the surface to be treated is contactedwith an aqueous acidic zinc phosphate solution containing fluoride, atleast one oxidizing agent accelerator, boric acid, and at least threemilligrams :per liter of an activating acting titanium phosphate.Desirably, the phosphatizing solution has a total P 0 content within therange of about 2 to 12 grams per liter and the solutions desirably alsocontain a nonionic wetting agent. Nitrate and nitrite ions are thepreferred oxidizing agent accelerators in the solution.

This invention relates to an improved process for coating ferrous metalsurfaces and more particularly relates to an improved method for formingthin, hard, tightly adherent zinc phosphate coatings on ferrous metalsurfaces, which coatings have excellent corrosion resistant andpaintbase properties.

Heretofore, zinc phosphate coating solutions have been widely used inthe chemical surface treatment of ferrous metals, such as iron andsteel, as a preparation for the subsequent application of paint orlacquer. The zinc phosphate layers produced by these processes are knownto increase the corrosion resistance and improve the adhesion of paintor lacquer films which are applied to the treated metal surfaces. Wherethin, hard zinc phosphate layers are formed, these have been found togive excellent adhesion values even where the painted part is subjectedto bending stresses.

Various modifications of a typical phosphatizing solutions have beenproposed in order to obtain this desired, thin phosphate layer. Thus, itis known that the addition of small amounts of a polyphosphate to anitrate accelerated zinc phosphate bath will result in appreciablereductions in the coating weight of the phosphate layer which isproduced. It is also known that similar results can be obtained by theaddition of large amounts of calcium to zinc phosphate baths whichcontain oxidizing agents. In both of these processes, however,difficulties are encountered in the analyses of the processing solutionswhich are necessary in order to maintain proper solution componentconcentrations and ratios. Thus, with both of these methods, thereplenishing of the operating solution is frequently effected largely onthe basis of estimation, which frequently results in a buildup of thepolyphosphate or calcium in the solution, both of which are detrimentalto the coatings produced.

It is also known, particularly with respect to coating aluminum andaluminum alloys, that thin, hard coating layers may be obtained by theuse of a hot aqueous coating solution containing monophosphate, anoxidizing agent, fluoroborate and an excess of boric acid. Frequently,however, the thin, hard phosphate layer produced by this method is notcompletely homogeneous,

particularly in areas where there are traces of dust, fingerprints, orabraded areas on the metal surface. At these points, there is oftenproduced a lighter colored coating having a different crystal structurethen that of the main coating and this has an unfavorable effect on asubsequently applied paint or lacquer, particularly on that appliedelectrophoretically.

It is, therefore, an object of the present invention to provide animproved process for forming thin, hard zinc phosphate coatings onferrous surfaces.

A further object of the present invention is to provide an improvedprocess for forming zinc phosphate coatings on ferrous surfaces, whichcoatings are substantially homogeneous and have excellent corrosionresisting and paint-base properties.

These and other objects will become apparent to those skilled in the artfrom the description of the invention which follows.

Pursuant to the above objects, the present invention includes a processfor producing a thin, hard, sustantially homogeneous phosphate coatingon ferrous metal surfaces which comprises contacting the ferrous metalsurface to be treated with a coating composition comprising an aqueousacidic zinc phosphate solution, which solution contains at least oneoxidizing agent accelerator, boric acid, fluoride, and anactivating-acting titanium phosphate in an amount of at least threemilligrams per liter, and maintaining the coating composition in contactwith the metal surface to be treated for a period suflicient to form thedesired coating thereon. In this manner, there is produced a very thin,hard zinc phosphate coating which is substantially homogeneous, andwhich has a very fine crystalline structure.

More specifically, in the practice of the method of the presentinvention, the ferrous metal surface to be treated, e.g., a surface ofiron or steel, is contacted with an aqueous acidic phosphatizingsolution containing zinc ions, phosphate ions, fluoride ions, anoxidizing agent accelerator, boric acid, and at least three miligramsper liter of an activating-acting titanium phosphate. Desirably, thetotal P 0 content of the solution is within the range of two to twelvegrams per liter. As is known in the art, phosphatizing solutions of thisgeneral type contain the zinc ions in the form of zinc dihydrogenphosphate and, desirably, have a pH Within the range of about 2.0 to3.5.

Various oxidizing agent accelerators may be utilized in the presentprocessing solution, such as nitrates, nitrites, combinations ofnitrates and nitrites, chlorates, bromates, and the like, as are knownto those in the art. Typically, these oxidizing agent accelerators arepresent in amounts up to about 3% by weight of the phosphatizingsolution, with amounts within the range of about 0.01 to 1% by weightbeing preferred. In many instances, the preferred oxidizing agentaccelerator is a combined nitrate-nitrite accelerator and reference willbe made hereinafter to phosphatizing solutions containing suchaccelerators. In such solutions, the nitrate ions are typically presentin amounts within the range of about 0.05 to 2% by weight of thephosphatizing solution while the nitrite ions are present in amountswithin the range of about 0.001 to 0.05% by weight of the solution. Itwill be appreciated, however, that as is known to those in the art,amounts of these oxidizing agent accelerators outside of these typicalranges may also be used.

The boric acid component of the subject phosphatizing solution isdesirably present in an amount of at least 0.5 gram per liter of H BOwith greater amounts, up to the maximum solubility of the boric acid inthe solution, being suitable. The boric acid may be introduced into thetreating solution as such, i.e., as H BO or it may be added as variousboron containing compounds which will form boric acid in the aqueousacidic phosphatizing solution. Exemplary of such boron containingcompounds which may be used are alkali metal borates, such as Na B O 1OHO and NaBO -SH O. The boric acid or boric acid forming compounds may beadded directly to the phosphatizing bath or they may be added to theconcentrate compositions which are used in making up and replenishingthe phosphatizing bath and, thus, added to the bath in this manner.

The activating-acting titanium phosphates, which are present in thetreating solution in an amount of at least 3 milligrams per liter, arecustomarily used in aqueous solution as a pretreatment for metalsurfaces before the application of a phosphate coating. Typically, thesematerials are compositions prepared from disodium orthophosphate and asoluble titanium compound such as titanyl sulfate. The processes for thepreparation of these activating-titanium phosphates are set forth inGerman Pats. 1,144,565 and 885,638. Although the activating-titaniumphosphate is present in the treating solution in an amount of at least 3milligrams per liter, greater amounts, up to about 1 gram per liter areoften typical. It is to be appreciated, of course, that in someinstances amounts of the activating-acting titanium phosphate in excessof these typical amounts may also be used with satisfactory results.

It has been found that by incorporating the titanium phosphate materialsin the phosphatizing bath, the rate at which the coating layer is formedis increased. Moreover, it is found that an improvement in the coatingitself is obtained which is in excess of that obtained by the individualcomponents of the phosphatizing bath, independent of theirconcentrations in the solution. Finally, it is found that a high degreeof uniformity of the phosphate coating is obtained when theactivating-acting titanium phosphates are included in the composition,the phosphate coating being formed uniformly on the metal surface evenin those areas which contain traces of dirt, fingerprints, or surfaceabrasions. It is to be noted, however, that inasmuch as the titaniumphosphates are colloidally suspended in the phosphatizing solution, itis desirable that replenishment of the activating-acting titaniumphosphate in the bath be carried out frequently or even substantiallycontinuously.

The metal treating solutions of the present invention also containfluoride, desirably in amounts within the range of 0.05 to 3% by weightof the treating solution. The fluoride may be present in the solution assimple fluorides, which are added as HF, alkali metal fluorides, or thelike, or they may be in the form of the more complex fluorides, such asthe fluoborates. As such, the fluorides may be added as fiuoroboricacid, or the alkali metal fluoroborates, or the like. It is to beappreciated, of course, that as is known to tthose in the art, thefluorides may be incorporated in the present processing solutions inamounts which are outside of the preferred ranges indicated above, aswell as in other forms which will not be detrimental to the processingsolution.

In many instances, it has also been found to be desirable to include anon-ionic wetting agent in the phosphatizing solution. In this manner, adegreasing and dirt removing action is imparted to the phosphatizingsolution, particularly when the solution is applied by spraying.Desirably, the wetting agent is present in the solution in an amountwithin the range of about 0.05 to 1 gram per liter of the phosphatizingsolution. Although various non-ionic wetting agents may be used, thepreferable wetting agents have been found to be the hydroxyethylatedalkyl phenols and straight chain fatty alcohols, particularly thosehaving an etherified terminal OH group in the ethylene oxide chain,which latter wetting agents are characterized by their low foamingproperties.

The phosphatizing solutions of the present invention may be formulatedusing any convenient source of materials which provide the desiredccmponents in the solution. Examplary of such materials are zinc oxide,phosphoric acid, zinc dihydrogen phosphate, zinc nitrate, and the like.Additionally, the nitrite ions, which are accelerators in the bath, mayconveniently be added as the alkali metal nitrite, such as sodiumnitrite. Moreover, as has been indicated hereinabove, boric acid may beadded either as such or as a boron containing compound such as an alkalimetal borate, which will form boric acid in the solution and thefluoride ions may be added as the simple or complex fluorides, either asthe. acid or in the form of fluoride salts. While other materials may beused to formulate the phosphatizing solutions, as are known to those inthe art, it is to be appreciated that in choosing these materials, thematerials used are desirably those which will not introduce extraneousions into the treating solution or, that at least will not introduceions into the solution which are detrimental either to the solutionitself or to the coating which is produced.

Desirably, the phosphatizing solutions are at a pH within the range ofabout 2.0 to 3.5 and are used at a temperature within the range of about40 to degrees centigrade, although operations outside of these rangesmay also be carried out in many instances. The phosphatizing solutionmay be applied using any suitable application technique, includingimmersion, flooding, spraying, and the like, although the advantages ofthe present process are particularly apparent when using flooding andspraying methods.

The phosphatizing solutions as described above, are brought into contactwith the ferrous metal surfaces to be treated, using any suitableapplication techniques, as have been indicated. The metal surfaces aremaintained in contact with the phosphatizing solution for a periodsuflicient to effect the formation of the desired phosphate coating onthe metal. Typical cotnact times which may be used are within the rangeof about 1 to 4 minutes, although contact times outside of this typicalrange will be used in many instances, depending upon the particularapplication techniques which are employed. After the desired phosphatecoating has been formed on the metal surface, the surface may then begiven a final rinse with a trivalent or hexavalent chromium-containingsolution, aqueous solutions containing from about 0.01 to 1% by weightof CrO either alone or in admixture with other acids such as phosphoricacid, being typical of the rinse solutions which may be used. Thethus-treated metal surface may then be given a protective coating of apaint or lacquer, which paint or lacquer coating may be applied byconventional dip, spray or flooding techniques, or by electrophoreticmeans, the conditions of such application techniques being known tothose in the art. The zinc phosphate coatings which are thus formed onthe ferrous metal surfaces treated are found to have a very fine-graincrystalline structure and are dark in color. Typically, the weights ofthese coatings are within the range of about 1 to 2 grams per squaremeter.

It is found that with the phosphatizing solutions described above, whichcontain the non-ionic wetting agent, the present process may often becarried out using fewer treating stages, because of the combineddegreasing and dirt removing action with the phosphatizing action of thesolution. Thus, in the first treating zone, the metal surfaces are bothdegreased and phosphatized, water rinsed in the second zone, and thengiven a final rinse or treatment with the aqueous acid solution ofhexavalent or trivalent chromium, in the third zone. In many instances,because of the excellent corrosion protection provided b the coatingswhich are produced in accordance with the present method, particularlywhen these coatings are used in conjunction with paint or lacquer film,it is possible to eliminate the final after rinse with hexavalent ortrivalent chromium, without any appreciable reduction in the corrosionprotection which is obtained.

It has also been found in using the phosphatizing solutions which havebeen described, substantially all of the grease and dirt removed fromthe metal surfaces is retained in the phosphate sludge which forms inthe coating solution. Thus, the solution containers and the walls in thephosphatizing zone remain substantially free of greasecontainingdeposits and the grease-containing posphate sludge is easily removedfrom the coating bath by filtration. This combined degreasing andphosphatizing also results in far less incrustration of spray nozzles inthe phosphatizing bath, as well as of the heat recorder, bath and tunnelwalls, as compared to that obtained in a sequential degreasing andphosphatizing process. Additionally, with this combined phosphatizingand degreasing action, any prepassivation of the metal surface isminimized. In contrast, this prepassivation is frequently encountered ina sequential degreasing-phosphatizing process, as a result of theinitial reaction of the phosphatizing solution mist with the clean worksurface as it is introduced into the phosphatizing zone. This reactionresults in the formation of thin, tarnish layers on the metal surfacewhich hinder the formation of a homogenous zinc phosphate coating duringthe phosphatizing step.

In order that those skilled in the art may better understand the presentinvention and the manner in which it may be practiced, the followingspecific examples are given. In these examples, unless otherwiseindicated, temperatures are in degrees centigrade and parts and percentare by weight. It is to be appreciated, however, that these examples aremerely exemplary of the present invention and are not to be taken as alimitation thereof.

EXAMPLE 1 An aqueous phosphatizing solution was prepared which containedthe following components in the amounts indicated:

Components: Grams per liter Zinc 3.15 P 6.75 N0 2.55 NaNO 0.15

This solution was then modified by the addition of the additive as shownin the following table, and the ratio of free P 0 to total P 0 wasadjusted by the addition of sodium hydroxide to the solution to thevalues shown in the table. Steel plates degreased in an alkaline spraycleaner were rinsed with water and then sprayed for three minutes withthese solutions which were at a temperature of about 60 degreescentigrade. The coating weights obtained in each instance and theuniformity of the coating are given in the table. It is to be noted thatthe activatingacting titanium phosphate, which is added to some of thesolutions, was a mixture of 90% of Na HPO and of titanium phosphate.

6 EXAMPLE 2 The steel plates which had been treated with solutions athrough k of Example 1 were then coated electrophoretically with alacquer and with a monolayer acrylate lacquer. The thus-coated plateswere then tested by bending over a conical mandrel and were alsosubjected to the salt spray test using the procedure ASTM Bl17-54-T. It7 was found that the lacquer adhesion of the plates which had beentreated with solutions a, c, d, e, f and g, was poor, in that thelacquer was loosened when using a bend mandrel diameter of from 10 to 30millimeters. Additionally, after 48 hours in the salt spray test, theelectrophoretically applied lacquer on the plates was loosened atdistances of from 2 to 3 millimeters from the scratch site while theacrylate resin lacquer, after 96 hours in the salt spray showedloosening of from 1 to 1.5 millimeters at the scratch site. The panelswhich had been treated in solutions b, h and i of Example 1 althoughhaving acceptable lacquer adhesion and corrosion resistance, evidencednonuniformity in the lacquer coatings, due to the poor uniformity of thephosphate coating which had previously been applied. In contrast, theplates treated with solution been applied. In contrast, the platestreated with solutions i and k of Example 1 were found to have excellentlacquer adhesion and corrosion resistance and, additionally, had asubstantially completely uniform lacquer coating.

While there have been described various embodiments of the invention,the compositions and methods described are not intended to be understoodas limiting the scope of the invention as it is realized that changestherewithin are possible and it is intended that each element recited inany of the following claims is to be understood as referring to allequivalent elements for accomplishing substantially the same results insubstantially the same or equivalent manner, it being intended to coverthe invention broadly in whatever form its principle ma be utilized.

What is claimed is:

1. A process for forming a thin, hard, substantially uniform phosphatecoating on ferrous metal surfaces which comprises contacting the ferrousmetal surface to be treated with a coating composition consistingessentially of an aqueous acidic zinc phosphate solution, having a totalP 0 content of from about 2 to 12 grams per liter, which solutioncontains at least one oxidizing agent accelerator in an amount up toabout 3% by weight of the solution, boric acid in an amount of at leastabout 0.5 gram per liter, fluoride ions in an amount within the range ofabout 0.05 to 3% by weight of the solution, and at least 3 milligramsper liter of an activating titanium phosphate Coating weight UniformityFree P205, in grams] of Additive and amount Total P205 sq. meter coatingNone 0. 12 2. 2-2.4 Poor.

4 grams/liter NazB 0 .10H20 0. 12 1. 3-1. 5 Do. one 0. 08 2. 6-2. 8 Do.4 grams/liter NazB O1.10HzO 0. 08 2. 4-2. 6 D0. e 0.2 gram/literactivating-acting titanium phosphate..- 0. 12 2. 22. 3 Average f 0.6gram/liter NaBF4 0. 12 2. 0-2. 2 Poor. g do 0. 08 2. 3-2. 5 D0; h. 0 6gram/liter NaBF4, 4 grams/liter NazB40 0. 08 1. 3 Do. ido 0.12 1.3 Do 3.0.6 gram/liter NaBF4, 4 grams/liter N82B4O1.l0H2O, 0. 08 1. 3 Good.

0.2 gram/liter activating-acting titanium phosphate. k do 0. 12 1. 3 Do.

From the above results, it is seen that only with solutions j and k,which solutions contain fluoride, boric acid, and the activating-actingtitanium phosphate, in accordance with the method of the presentinvention, are the desired thin, hard zinc phosphate coatings obtainedwhich are substantially uniform in nature.

and maintaining the ferrous metal surface to be treated in contact withthe solution for a period sufiicient to form the desired coatin.

2.. The method as claimed in claim 1 wherein the oxidizing agentaccelerator in a combination of nitrate and 7 nitrite ions.

7 8 3. The method as claimed in claim 2 wherein the phos- ReferencesCited phatizing solution has a pH Within the range of about 210 UNITEDSTATES PATENTS to 2,322,349 6/1943 Iernstedt 148-6.15 4. The method asclaimed 1n claim 3 wherein the con- 2,479,564 8/1949 Gilbert 4g 5 tactof the treating solution with ferrous metal surface 5 3,090,709 5/1953 Hick 148-6.15 is effected by spraying the solution on the metal surface.3,420,715 1/ 1969 Ayres 1486.1S X

5. The method as claimed in claim 4 wherein the oxidiz- FOREIGN PATENTSing agent is present in an amount within the range of about 0.01 to 1%by weight, and the titanium phosphate is 10 present in an amount up toabout 1 gram per liter. R ALPH KENDALL, p i Examimr 655,079 7/ 1951Great Britain.

